The invention relates to a method and a circuit arrangement for converting Gray code signals into counting pulses, forming a counter status, detecting interference signals infringing the Gray code, and forming an error signal.
Gray code signals are frequently obtained in precision dimension measuring machines from scale scannings via the intermediate stage of sine and cosine signals, are transferred to an evaluation device and processed there via a counter to a coordinate specification as a measurement result.
In accordance with the precision requirements of dimension measuring machines, it is important that the indicated measurement result is not falsified, for example, by interference signals coupled in electromagnetically from the environment. Interference signals which clearly deviate with their temporal course from the measurement signals, in particular from shorter pulses, can be filtered out. This function is performed in a quadrature decoder HCTL-2000 produced by Hewlett Packard (see Hewlett Packard, HCTL-2000 Technical Data April 1986).
Interference signals with a time behavior similar to the measurement signals on only one input line also usually pose no problem if their switch-on and switch-off edge simulate an incrementing and a decrementing event which when summed amount to zero and hence remain without effect.
However, interference signals are frequently impressed simultaneously in parallel signal lines for the Gray code signals. This infringes the definition of Gray code signals as stated in U.S. Pat. No. 2,632,058 issued to Gray, according to which two (or more) signal levels may never change on the parallel signal lines at the same time, in practice within a defined time interval.
Even signals which are only impressed on one input line may lead to a Gray code infringement if with one of their signal edges coincide temporally with an edge of a measurement signal on another line.
A circuit arrangement for converting Gray code signals into counting pulses and for forming a counter status is known which outputs an error message in every case where a change of the signal level occurs simultaneously on both inputs. See, for example, "Simplified Multiplier Improves Standard Shaft Encoder" by Michael M. Butler, Circuits and Software for Electrical Engineers, McGraw-Hill 1983, p. 86. Two logic square-wave signals which are derived from a scale scanning and are phase-shifted by 90.degree. are stated as input signals. Each signal change of a signal level results in an incrementing or decrementing pulse, hence the circuit arrangement is also a quadruple interpolator.
In this circuit arrangement, the incrementing and decrementing pulses are not reliably separated temporally, which may result in malfunctions of the counter. For each error message of a Gray code converter, a measurement operation of the measuring machine must be terminated and begun anew, which is very costly in terms of time.
When an interference signal which infringes the Gray code is temporally separated from the level changes of the measurement signals, that is to say when following a simultaneous level change on two input signal lines the next level change again occurs simultaneously on these two input signal lines, then this double Gray code infringement can clearly be differentiated from a measurement signal as interference. The forming of counting pulses can therefore be suppressed and an error signal need not be formed.
If, however, a level change of a measurement signal occurs during the duration of an interference signal on two input signal lines, then this is "swallowed". The level is, however, reset on one input line at the end of the interference signal, and on the other at the next level change of the measurement signal. However, on which input line the level is reset by the interference signal and on which it is reset by the measurement signal, depends on the unknown temporal extent of interference signal and measurement signal. It is then undefinable whether an incrementing or a decrementing event is present and, in this case, an error signal must necessarily be generated.
Gray code infringements are also possible as a result of interference signals on only one signal line if by chance one edge of an interference signal temporally coincides with the edge of a measurement signal on another line.
In this case, no error message is required if by chance coincidence of the second edge of the interference signal with a second edge of the measurement signal a double Gray code infringement arises. This is only possible if the two edges of the measurement signal would have to trigger a pair of incrementing and decrementing pulses with the sum zero, so that as a result no counting is falsified.